Vibration velocity measuring device



'May 21,- 1935.

C. D. GREENTREE ET AL VIBRATION VELOCITY MEASURINGDEVICE Filed Aug. 25, 1953 hvehtors' Charles D.G-eeht`ee,

f n n.

maar may zi. 1935 j' viaa'rlon nevica 'chai-ies n. Greenfee ma ,Milton s. Mem, Jr.,

Schenectady, N. Y., assig'nors toGeneral Electric-Company, a corporation 'of New York yApplication August 23, `19332 Serial No. 6816.404

l14 claims. (c1. rarasn '35 -quency range. l

Our invention provides a device of this char" Our invention relates to devices which are caf v pable ofconvertng vibration energy into another form of energy; e. .g electrical energy, and vice` versa. The/.principal object of 'our invention is to provide an improved device of the above-men-` tioned character.-A A .more specific object yof our invention is topprovide an improved devicelfor lranotasuring the vibration velocity of'a vibrating The steadily growing interest in the reduction of noise levels in machinery, factories, olcesnand homes is causing engineers and scientists to .make

an 'intensive study of noise problems and noise suppression. I 'he amplitude atwhich a given object vibrates depends largely upon its physical characteristics. whereas the frequency of its vibrationdepends largely upon its physical characteristics and upon'the fundamental frequency of the vibration source; and, therefore, the .determination ofthe amplitude and frequency is usually of interest only to the designer. The velocity at which an object vibrates, however, is of prime interest in a consideration ofnoise measurement or noise suppression. 'I'he reason for this is that the noise produced by a vibrating ob ject is a measure of the energy it expends, and the vibration. energy it expends is more often proportional to the velocity of'vibration than it 1 is either to amplitude or frequency of vibration,

because the vibrationv energy is substantially directly proportional to thel square of the velocity. It, therefore, became desirable-to provide a device which is capable of measuring thevelocity at which a body vibrates `in the audible freacter. Briey described, a preferred form of our .device' consists' of a movable magnetic field struc'- ture adapted to be gripped by the hand of an 40 operator and arranged' to provide anair gap as' a part of its magnetic circuit, a movable rod hay-v ing one end-thereof adapted to make contact with the vibrating 'body whose vibration velocity is to be measured and having secured to its otherend' a coil disposed in the magnetic air gap, and spring -means interposed between the vmagnetic `eld 1 lstructure and rod toproduce an elastic restoring force therebetween and for restricting the rod to 'Y degree of freedom that causes the coil to `cut the magnetic ilux'passing across the -airgap in response to relative motion between the rod and the magnetic field lstructure. The mass and sti'i- 'ness of therod and parts secured thereto are so selected that this structure has a narrow resonance range and sothat its-natural frequency of vibration is considerably'above .the lowest frequency -of vibration of the vibrating and preferablymls appreciably above the highest frequency of vibration of the vibrating body, whereas the mass of the magnetic field structure and I 'the stiffness of the springs are so selected that the natural frequency of vibration of the :field `structure withvre'spect to the above-mentioned rod is' appreciably below the lowest frequency of vibration of the vibrating body. The structure l0 comprising the rod and the parts'secured thereto, therefore, vibrate in substantial synchronism withv the vibrating body, whereas the magnetic field structure remains almost stationary-over the entire frequency range at which the vibrating body vi- 15 breites, hence the relative motion between the: coil andmagnetic field structure is'substantially di- 4 rectly proportional tothe vibration velocity of the vibrating body. It, therefore, follows that the.

voltage induced in the coil will be substantially .20

directly proportional to the vibration velocity of the vibrating body. The voltage induced in the 'coil,.or a voltage substantially directly proportional thereto, is impressed on a suitably calibrated instrument 'whichindicates the vibration ve- 25 locity of the vibrating body.

Although', when any of^the commonly used ,forms of springs is interposed between the rod and the magnetic eld structure in the abovedescribed'vibration' velocity measuring-device, 30'

the latter can be readily built to measure with a high degree of accuracy, the vibration velocity `of a body when the latter vibrates at or near the l y highest frequency ofthe audible frequency range,

or even above this highest frequency, and to 35 quency` of the audible frequency range. 'The rea- 45 son'for this is that in order to obtain the desired measuring accuracy at or near the lowest frequency of the audiblefrequency range it is necessary to have the natural frequency of vibration of the magnetic fieldlstructure appreciably below 50 this lowest frequency in order that -this structure .will remain practically stationary while the rod is vibrating substantially in synchronism-with the vibrating' body. This, howevenmakes it necessary to employ. a` spring or'springs between 5-5 relatively low-dynamic force, i. e., a spring re- I quiring a comparatively small change in force to produce a comparatively large change in deilection thereof, in other words, a spring having a comparatively stillness. n the other hand-,.mechanical considerations of construction require that this spring also have a comparativelyhigh static force, i. e., have a comparativelyhigh total force. In order to satisfy both of these requirements it is necessary, when employing any of the commonly used forms of springs. to use a comparatively long spring. This, however, is a serious disadvantage tothe practical success-or a device of this 'character because it increases the size and cost thereof andv makes it ditiic'ult,

if not impossible, to build the device as a portable 'measuring devicek which can be readily' gripped by thehand of an operator. v

Our invention; however, overcomes this ldis- *advantage by interposing between the rod and magnetic field structure one or more comparatively short' springs each of .which has such peculiar e comparatively long commonly used form off shrines. "This additional and highly important feature of our invention,gtherefore, makes it possible to provide a device having a highmeasuring' accuracy' not only- `in the frequency range but forcecl'eiiectionv characteristics -that over a critical operating range thereof it has a comparatively high static force but a comparatively low dynamic force, in otheiwordait hasduring this critical operating range the characteristics of a comparatively yioni; commonly used forml or spring. we also-provide 'means for limiting the relative mo- -tion between the rod and magnetic field struc- Vture-to corinne the operation of each of springs to its above described eric/ical operating range; 'We are thus enabled to Aemploy one or more comparatively short prima and yet obtain a device which" operates as if it hadone or more also above and below this range, and at the same time the `device can be made comparatively small so as to be readily portable and lbe easily gripped by ,the hand ofl an operator.

our invention. however. will be beat understood from the following'desciption when coin-v 'sidered .in connection with the accompanying while those features or our-'invention which are, believed to be novel and-patentable are pointedout in` the appended claims.

Fig. l of the drawing represents a sectional of ii;A preferred form of, our device employl ing-only .t`wo comparatively short springs each "having the peculiar force deflection characteristics previously mentioned. This figure diagrammatically represents the lcoil of our device con- .i soV with four springs ofthe commonly used type.

. represents a sectional end view taken on the vline 5-5 .in Fig. 4. Fig. vil represents a side-view of a portion of our device showing the contact rod havinga-a'nged plate secured thereto for. making contact with the vibrating body-whose vibration velocity is to ybe measured. Similary parts in the various figures are represented by similar reference characters.

Either form .of our invention can measure the 2,002,372 the rod and magnetic field structure having a vibration velocity .of a vibrating body, and can measure. this velocity in any one plane at a time. For the sakeof illustration welhave represented in Fig. 1 a portion of ayibiating body by reference numeral I0, this body portion vibrating in the plane represented by the' double headed arrow II.. The device shown in Fl'g. 1 formeasuring the vibration velocity of body Il'consists' of a cylindrical casing, I2 of magnetic material and of suitable external diameter to be readily gripped by the hand of an operator and-'having secured to its internal periphery. by any suitable means (not shown) anannular ring I 3v of magnetic ma terial. Secured to the right-hand end of casing I Z by screws I4 is a cover I5 of magnetic material and secured to this cover by screws, is a magnet I1, preferably a permanentmagnet. 4Magnet .I'I is cylindrlcaliy shaped, its free end has an external diameter slightly smallerthan the internal diameter of ring il, and the length and arrangement of the magnet are such that there is an annular air gap between the free end of -v the magnet and ring Il. hence a' magnetic flux passes across this air gap. Pressed Into the free end of magnet I1 andinto plate II, respectively.

are -two suitable antifriction self-lubricating' bearings l l and Il respectively, and slidably' mounted in these bearingsis a light-weightmetallic contact rod 20 for receiving or delivering the vibration energy, depending on how our device isto be used. A portion of thisrod extends away from plate Il and has itsfree-end' adapted to make contact with some part of vibrating body III, as shown, and to the other end of 'this rod is secured a coil holder 2l towhich is secured a coil' 22 infthe gapl between the free end of magnet I1 andring I3.

Secured to the left-hand end of casing I2 by screws!! is a plate 24 of magnetic material and secured to this plate by screws 2l is a cup-shaped cover 2|. To the inside face of ,plate 24 is secured a vplate 21 having two )l-shaped notches 28 which are more clearly illustrated in Fig. 2.. Two simi- 'lar notches are provided-in coil holder 2 I [notches are substantially Ythe same distance from the of rod 2 0 and are so positioned that one of the notcheson plate 21 is substantially directly opposite one of the nphes on coil holder 2 I and the other notch on platen is substantially .di-

rectly opposite the rother notch on coilholder 2 I. Y Two comparatively short steel column springs 2l" having V -shaped-ends are interposed between -coii holder '2l 'and plate r1 to produce en ei'astio restoring force therebetween, lthe ends,of one spring iltting into two of the substantially opposite notches 2 8, and the ends of the other spring .mung ini-.o u; :otneit'wo substentiauy opposite notchesl.4 clearly4 shows how one. of the V-shapei ends'of a spring .2Q ts into one of the notches il, and from'this figure it is clear that the notch is sufficiently large to permit considerable change in de'nection -of the spring without the latter striking" either side of the niotch.V

Each spring has the peculiar force-deflection characteristic represented bythe curve shown in lflg. 3.l Thus, spring has Va substantially uniform fstiilness fromits free state represented by A until it'has been deflecteda critical amount as represented for example by B. By the spring stiffness 'we mean the number of pounds force required'to changev the deflection of the spring a unit amount. As soon as the spring is deiiected above its critical deilection value, itsstiifness almost abruptly decreasesto a considerably lower value and practically remainsat this lower value 2o y vantage .of the .unique characteristics'- Aof the ispring, and we are thus enabled to employ two with further denetiqns thereof until its' elastic- 'limit-is reached, as shown for example by C. The

great difference between the stiffness of the spring before and'after it has Qbeen deflected to its critical deection value may be readily seen from the great difference between vthe slopes of the lines ating range the characteristics of a comparatively long commonly used type `of spring. We confine the deflections of each spring during theoperation of our device between vitscritical -deilection value and-'its eiastic limit/in order to take. ad-

springs which are comparatively short but which,

nevertheless, have thestiiness characteristics of two comparatively long commonly used type of springs. y

The marked.change in slope of the force-de` ection diagram 4from that in the initial portion of the curve n earthe origin -A to that in the operating portion ofthe curve between D and E results, we'believe, from the fact that, at first, the spring acts substantially as an undeflected column subject to compressive force only as in a short block or pier. With the application of greater. force, however, as the springdeflects appreciably to the side or laterally, bending stresses come into playand the force per unit area in the outer fibers of the spring may-be much greater .than

' the quotient of the totalqforce acting and the so that the moment arm vof the external force cross-sectional area ofthe spring. Increased def lection increases the eccentricity of the loading increases .nearly as rapidly'as the rsisting'moment-in any section .of the spring. Consequently, in the operating portion of the force-deflection diagram, relatively large variations in deflection correspond to very slight ,variations in external force. 'I'he portion of the force-deflection dia- Y gram, Fig. 3, beyond E, inrwhich force decreases with deilection corresponds to bending the spring beyond its elasticlimit, after which the usual relationshipv between uniti-stressv and deformation of elastic material no longer holds, as isf-.well

- knownin the a'trf' 'We confine the deflection of each spring to its critical operating. range by securing to plate.

21in Fig.l a rod 30 of vsuch'length and by making bearing i8 of such. length that when coil jholder 2i rests against the bearingA each )spring is deflected an amount represented by F in Fig.

' 3, for example, vand when ythe coil-holder rests.,

against thefree end of the rod 30 veach spring is deflected an amount represented by -H in Fig. 3, for example. .The ends of coil'22 are connect-l ed by means olf/leads passing through holes in l plate 24 and cover 26 to the winding of a transformer 3l whose secondary winding isk .connected in series with a full wave rectifier 32` to an electrical indicating instrument 33 whose I scaie'isA preferably calibrated in vibration velocity mils per; second. y

' Rod 20, coil holder. 2i and coil 22are sonproportioned and dimensioned that the mass of this structure and the stiffness of the rod (it being remembered that the rod issomewhat elastic) are contactl rod against the vibrating body such that the natural frequency of vibrationl of this structure is considerably above the lowest ing rod 20, coil holder .2l and coil 22 is made ap- I preciably above the highest frequency of the .audible frequency range or atleast above the highest frequency at which body I0 vibrates.

The .mass of casing I2, magnet .I1 and all other parts-secured .to the casing and the 'stiffness of each spring `29 during* its critical operating range are such that theV natural frequency of 'vibration of the casing and magnet is appreci-f 'ably below the` lowest frequency of the audible range.- By employing column. springs 29 in the manner ydescribed it is readily possible tomake the `natural frequency of vibration of casing i2 and magnet i1 as low as 2 or 3 cycles per second,

whereas such a low natural frequency would be l'extremely diilicult, if not impossible, toobtain if any ofthe commonly used formsof springs -were used in place of springs 29.

Asssume, for example, that the audible Ifrequency range extends from 20 to 15,000 'cycles per second 'and that body I0 may vibrate at any frequency from 20 to 3000 cycles per second. When an operator desires to measure the vibra.- tion velocity of body l0 he grips casing l2 in his hand and presses the free end of contact rod 20 against thevibrating body with sufcient force to move-coil holder 2l -away from bearing i8 so the coil holder will be' practically .central berod 30, the contact rod 20 will have .oscillatory 'tween this bearing and the end of rod 30. As- -suming that the amplitude of vibration of body l0 is not sumclent to causethe lcoil holder to strike either bearing i8 or the adjacent end of translational motion in substantial synchronism with body 'IB because springs 29 will keep the Furthermore, since-the structure comprising rod 20, coil holderA 2i and coil 22 has a natural frequency of vibration which is appreciably Aabove the4 highest frequency at which body i0 vibratjes and has a very narrow resonance band in the vicinity -of its natural frequency of vibration, this structure. will vibrate at substantially the saine velocity as body l0 throughout the range of frequencies at which the latter vibrates. Casing i2 and magnet i1, however, will remain practically stationary during the entire vibration'A frequency range of body I0 because the natural frequency of vibration of the 'casing' and magnet is appreciably below the lowest frequency of vibration of body l0. Consequently,".

the velocity of the relative motion between casing I2 .and rod. 20 will be substantially equal to the vibration veicity. of= body lo during the Y entire, vibration frequencyk range of thelatter, hence 'during this frequency range coil 22 will cut the magnetic flux passing across the annular air gap ata ratewhich is substantially directly proportional to theV vibration velocity of body I0 as.v the lcoil oscillates a comparatively small amount in a-concentrated magnetic eld. It, therefore, follows that the voltage-- induced-in coil 22 will be Substantially directly proportional to the'vibrationvelocityof body I0 throughout GIA' body I in mils per second.

It is obvious that our device will also accurately measure the vibration velocity of lbody I0 when thev latter is vibrating below the lowest frequency oi the audible frequency range, providing, however, that the body is vibratingappreciably above the natural frequency of 2 or 3 cycles per second of casing I2 and magnet I1. Also, assume that body I0 may vibrate above 3000 cycles per second and up to the highest frequency of the audible frequency range and even higher, and further assume that it is desired to measure the vibration velocity of this body. Our 'device will then even under the most unfavorable conditions accurately measure the vibration velocity of body III except during a very narrow range of frequencies. Thus, assum'e for example that the structure lcomprising rod 20, coil holder 2l and coil 22 vhas a natural frequency of 5000 cycles per second. However, since this structure l has a very narrow resonance band in the vicinity of its natural frequency of vliration, it will vi.-

brate at a velocity which is substantially 'equal' to the vibration velocity of body I0 except when the latter is vibrating at 5000 cycles'per second and during a few cycles on either side of 5000 -cycles'per second. In view of this fact and the further fact-that casing I2 and magnet I1 will remain practically stationary Aduring the entire vibration frequency range of body III, it follows that,

our device will accurately measure the vibration 'velocity of body III' except during a very narrow g frequency range of its vibration. It should be clear; therefore, that our device is not only ca-l pable" vof measuring with a high degree of accuracy the vibration velocity of a body vibrating inthe 'audible frsquencyvrange, but will also do this both above and below the audible frequency range.

The total 'impedance of coil 22 and transformer 3l. is approximately matched againstv the total impedance of the output circuit consisting of rectifier 32 and instrument 33. This makes available the maximumelectrical output from our.,

device 'for operating an electrical instrument,l hence making it possible to reduceour device to the smallest possible size that will produce the desired result.

vCil

The device illustratedfin Fig. 4 consists of a ,Y

cup-shaped cylindrical casing 34 of magnetic material and of suitable external ,diameter to be' readily gripped by the hand of an operator and having secured to its internal periphery an an- .'/nular ring I3 ofwmagnetic material. Secured to the inside bottom face of casing 34 by means of; a screw 35 is a magnet I'1,` preferably a penna-- nent magnet. This magnet is of such shape and dimensions that there is an annular air gap be` tween the free end of the magnet and ring I3, hence a magnetic'ux passesacross this air'gp. Secured across the otherwise. open end of casing 34 is a plate 36 of magnetic material having a proximately their` center to rod 2Il so that they I are substantially perpendicularto each other land having their free, ends secured'tobosses on the internal periphery of casing 34 by screws 38. This arrangement of springs 31 will be easily un-r, derstood from Figs. 4 and 5. A coil holder' 39 is secured to the end of rod 2l adacent magnet I1, and secured to this coil holder is the coil 22 disposed in the annular air gap between magnet I1 andring I3. It is clear that although there are f no guide bearings in which rod 21| can slide, the arrangement of springs 31 as shown will restrict the rod to a single degree.of freedom that will l0 cause coil 22 to cut the-magnetic ilux passing across the annular air' gap when there is relative motion between the rod and casing. Secured lto rod is a plate Il Lhaving two V-shaped each spring 2l nts into ary-shaped notch 28 as shown in Fig. 2. A collar II is secured to rocly 20 a shortdstance away from the inside face of plate 30, the outside diameter of this collar be- -ing larger than the diameter of the adjacent holev 25 in the plate.

We will first describe the'operation of the device s'hown in Fig. 4y when column springs 29 are omitted therefrom and then describe the advantage 'of egnploying these springs 'in addition 4to 30 springs 31. Assume that springs 29 are omittedfromthe device shown in' Fig. 4. The operation of the device willv then be similar to that of the device shown in Pig. 1 except that the natural frequency of vibration of its. magnet and casingwill be `appreciably higher than'that of the magnet and casing of the deviceshown in Fig. 1. The reason for this is 'that eachcomparatively short commonly used type of springs I1 -aw has a much higher stiffness than 40 that whi can 'he obtained from -each column spring in Fig. 1. when the latter is operated .over its critical operating range as previously described. Consequently, with column springs 29 omitted the casing and magnet of the device shown in Fig. 4 will not remain practically stationary to as low a frequency of vibration of body Ilaswill the casing 'and magnet of the device shown in Fig. 1, and, therefore, will not accurately measure the vibration velocity of body l0v 5o to 4as low a frequency of vibration as will the `device shown in Fig. 1. In many cases this-'is not a disadvantage because it is-frequently `desired to measure the vibration velocity of a body vibrating through a range ofA frequencies the lowest of vwhich is appreciably above the naturalr `frequency of vibration of the casing and magnet of the device shown4 in Rig. 4 when its column springs 2l' are omitted. It is, therefore, ,clear that the device shown in Fig. 4 with its column (i0 v springs 23 omitted will have an important field of practical use. I

When, however, it is desired to employthe device shown in Fig. 4 and to measure accurately therewith the vibration velocity of a vibrating 4 body down -to a frequency which is lower than that which can be accurately measured by the. device lwith its column'sprlngs .-29 omitted,- then this may bp accomplished by also employing its' columnsprings and the deflection of .70 each' column spring during-the operation ofthe device to its criticalijoperting range.' This is obtained havlnthe end of.rod 20 inside-'of casing- 34 the proper distance from theadjacent faceofmagnetHandhavingcollarM theproper I aooasva distance from thedface of plate 36 so that when the collar rests against the plate each column spring29 is deflected to lthe value yF jin Fig. 3 and.when the end of rod20 rests against the magnet each column spring 29 is deiiected to the value H in' Fig. 3. "Iheii'esultant stiness lof the combination oi' springs 31 and that of each \column' spring 29 vduring its critical operating range will cause the casing and magnet of the device shown in Fig. 4 to havejan appreciably lower I natural 'frequency of' vibration than it would have with column springs'29 -omitted but it will still be appreciably higher than that of the' casing and magnet of the device shownin Fig.l l, assumingof course that the mass of the casing and magnet and .the partssecured to the casing is the same in Figs. `1 and 4. It is, therefore, evident that the Adevice shownin Fig. 4 has the advantage of being capable of accuratelyA measuring the vibration velocity of a vibrating body down to a frequency which is lnot very much higher than the minimum frequency at whichv from magnet-l1 serves asa magnetic shield,A

thus eliminating possible errors in measurements due to any -external stray flux in the vicinity of the device. l

A very useful -iield of application forl either form of`our-device will ,be as a Go or No Go measuring device. AThis eld of application may be illustrated by assuming a. productionl line of 5,- for example.

vrieedsaio outside source-of power for its operation,

electric or internal` combustion motors going has been measured for its vibration velocity in many different planes -and that the maximum indication of instrument 33 during this .test was The motors inthe production line are then tested one after another for Vvibrationjvelocity in different planes.' This can be readily accomplished since instrument 33,' rectiiler 32 and transformer 3| are so smallthat they "can be held in one hand, especially when theyare assembled as Aa unit, and the tester grasps the casing of the device in the other handyandsince the entire device is light, portable, and

the tester can rapidly test .each motor `in many diderent planes. .When a motorvibration produces a reading of 5 or under. on instrument 33,

owne tester knows that this motor is satisfactory,

' tion produces a reading 'of more than 5 on the unsatisfactory for noise and he returns for production use, whereas vwhen a motor vibrainstrument, the tester-knows that the motoria assembly room for, correction. 4

.Y It is bration-velocity of a vibrating body whose. physical nature is such that it is undesirable to press thereagainst the small free end of the .contact rod, or whose expenditure of-vibrationenelgy Der unit- -of 'area Yis so small that an insufficient amount of this energy will be transmitted to the rod to obtain satisfactory operation of the'meas- 'uring device. lvAnexampleof this-is when itis desired to use -either -form of our device.- inja through test, andassuming that an acceptable: motor, in so far as the noise produced thereby,

it to the ftentimes necessary to measure the vi-` manner for whichestethoscope is used,.ex'cept` that instead oi.' listeni'ng for sounds produced in the patients chest, abdomen, etc., it is desired to measure the velocity v'oi the vibrations which produce these sounds. The aforementioned. diiliculties may be overcome -and satisfactory operation of the' device obtained when used withvibrating bodies of. this character by securing, as shown in Fig. 6. a. flanged member l2 to rod 20by a screw 43,"

the contact area of the flanged member being considerably greater than the area of the rod end.

In accordance with the provisions of the patent statutes, we have described the principles yof operation of our invention, together with the apparatus which we now consider -to represent .the bestv embodiments thereof, but we desire to have it understood that the apparatus shown land described is only illustrative and that the inven tion mayI be carried out by other'means.

What we claim as new and desire to-secure vby Letters Patent of the United States is:

1. In a device for converting vibration energy into another form vof energy and vice versa, the combination ofia'movable structure arranged to receive or deliver said vibrationenergy, another structure, means operatively associatedwith said structures for effecting said energy conversion vin response to relative motion between said structures, resilient 4means interposed between said structures for producing an elastic restoring force therebetween, said resilient mmns having a critical operating rangev during-which its forcev I .deection ycharacteristic diifers radically from .the force deiiection characteristics it has outside this criticaloperating range. 'and means Ifor limiting the relative motion of said structures to coniine the operationof said resilient means tov its critical operating range, the relationship bef l 'tween thermass oi' the second-mentioned structure and-the stiffness of said resilient means during its' critical operating range beingsuch that the natural frequency of vibration of thisA structure is.appreciably below the lowest fre-Y quencyuoi? said vibration ienergy', whereby only a negligible vibration force is transmitted by the mst-mentioned structurev to the second-melitioned structure during the frequency `range of 'l said vibration energy. 2. .In a device for converting vibration energy into electrical energy and vice versa, the combi-.

. nation of a movable structure arranged to receive or deliver said vibration energy, another structure, one of said structures comprising-a magnetic eld system having an gap as a part of its toa relative movement of 'said structures, resilient means. interpom beween said structures for producing an elastic restoring force therebetween', said resilient means'having a vcritical operating range during which its force deflection characteristic differsv radically' from the Torce deiiection characteristics -it has outside critical operating range, means for limiting the relative; Amotion of said structures to conne the operation -of said resilient means to its critlal'operating range,`therelationship. between themass of the second-mentioned structure andthe `stiimris of Isaid resilient meanslduring its critical operating range being such that the frequency or 'vibration of this strlicture is appreciablyv below the'lowest `frequency of said vibration energy,

wherebynly negligible lforce istransmagnetic circuit and the other of .said structures comprising a. coil disposed in saidair gap to eut. the magnetic flux passing .thereacross in response cls \ range of said vibration energy.

lower amount for de'ilections thereof above this the operation of said springmeansto those deiiections at which it has said considerably lower-suitstructure, means structures foreiecting said energy conversion 3. In a device for converting vibration energy 'into electr-ical energy and vice versa, the combination Iof a movable structure arranged to receive .or deliver said vibration energy. another struc- `ture, one 'of said structures comprising a magnetic iield system having an airgap as apart o! its magnetic circuit and the other of comprising a coil disposed in saidto cut the magnetic flux passing thereacro'ss in response to a relative movement of said structures, Spring means interposed between said stnrchrres f or producing an elastic 'restoring force therebetween,

said spring mnsnvinga.. critical-resection value at which itsrstilne almost abtlllltly'decreases from a'glven amountto a considerably citicaljdeilection'value. and means for limiting the relative motion ot structures to-conne ness, thefrelationsbipbetween the mass of the second-mentioned structure and the stillness of the spring means during itsconnnedoperating range being such that the naturalfrequency of.

vibration of' the--second-mentioned structurels appreciably belowthe lowest frequency of said -vibration energy, whereby only a neslisibl Vibrav vtion force is transmitted by thevrst-mmtioned structureto the second-mentioned structureduring the frequency f said vibration energy, 4. Ina device for converting vibration energy combination cfa' movable structure arranged to receive or deliver saidvibration energy, another operatively with said in-response to relative motion between saidv structures,'at least one spring interposed betweerr said structures'for producing an elastic restoring force therebetween, -said column spring having two critical deflection values between whichv its-stiffness is comiderably lowerthan that 'from 11s free me to the lower oros ness changing .abruptly at ythe lowero! the two critical deflection values, and means for glimiting'the relative motion o! said structures to corinne -the deilections'of said spring between its critical deilection values, the between the mass of the second-mentioned structure and the stillness of said spring between its critical deflection values being irequency ofA vibration of the second-mentioned structurels appreciably below the lowest frequency of said vibration energy, Awhereby only a negligible vibration force is transmittedby the inst-mentioned structure to the second-mentioned structrn'e during the frequency' range o said vibration energy.A

' v.5. In a device i'orconverting vibration into' electrical energy and vice versaythe combination of a movablest'ructure arranged to reoeive or-deliver said vibration-:emersa anotherstructure. oneolsaidstructures oompridng a a'eoildisped in saidair aooasvz deflection values, the relationship between theA mass of the second-mentioned structure and the stine'ss ofv said spring between its critical deflectionvalues being such that the natural frequency of vibration of the second-mentioned structure is appreciably below the lowestfrequency of said vibration energy, whereby only a negligible vibration torce is transmitted by the inst-mentioned structure to the second-mentioned structure during the frequency range of said vibration energy. 6. In a device for converting vibration energy into. electricalA energy and vice versa, the combindition of aj movable structure arranged to receive 0r deliver said vibration energy,` another structure, one of said structures comprising a; magnetic eld system having an air gap as apart of its magnetic .circuitand the other -of said structuresI comprising a coil disposed in said air gap,

ture to adegree of freedom thatcauses said coil.

to 4outl the magnetic uir passingjacross said air, gap with relative movement of said structures, at

spring means vinterposed between said structures least one column spring interposed between said structures .for producing an additional elastic re--I into anothergform of energy and vice versa.' the storing force therebetween, said column spring having two critical deflection vvalues between whichfjitsjstiiness is. considerably lower than that prevailing from its free state to the lower of its two'critical deiiection values, said sp'ring'stiffness changingalmost abruptly'at the lower of the two critical'deiiection values, and means -for limiting the relative motion of s aidstructures to Acon- -flne the deilections of said column springl between its critical defiectionvaluesthe .relationship be'- tween the mass ofthe second-mentioned structure, the stiiness of the rs't-mentionedspring tween its critical deflection values being such that means-'and the stiiiness of said column spring be- "the natural frequencyof Vvibration of this struc- 1 tureis appreciably'below the lowest frequency of said vibration energy; whereby only a'negligible vibration force is transmitted 4'by the rst-mentioned to the vsecond-mentioned structureduring the frequency range of said vibration energy.

7. A device for converting'mechanical vibrationsjinto electric voltage comprising a. ,movable" structure adapted to `be subjected tothe 'vibrations tobe investigated and-to move in sub-f:

f stantial synchronism with such vibrations, a seci ond movable structure, means 'operatively asso- Velated with said structures for generating an electrical voltage iii-response tofrelative. motion between the structures, at least'one columnspring between saidstructures forproducing an elastic restoring-force therebetween; said colwhich its stiiiness is considerably lower th'at prevailing from its tree state tothe .lower of its two'critical 4deileciion values, said for the relative motion vof said spring stiitness changing almostabruptly at jthe lower of. theltwo critical jdeectionwalues, andm critical deflection-values.. 15

umn spring having two critical. deiiection values lrange ofsaid vibration energy,

mentioned structureand the of said.

column spring between its critical de ection values bein'gsuch that the natural frequency 'of vibration of this structure is appreciably belowr thel lowest frequency ofsaid vibrationjf'energ'y,

,whereby the second-mentioned structure remains guiding said movable member so that'th 'latter f has oscillatory (translational 'motion in substantial synchronism with that partof 'the vibrating body itis in contact with, a movable magnetic iield structure having an air gap as a part o f its vmagnetic circuit,a coil secured to said movable member and disposed in 'saidair gap to cut the magnetic flux passingthereacross in response to relative motion Zbetween themovable memberv and the magnetic eld structure,l at least one column 'spring-interposed between said'movable lowest frequency` of vibration of said vibrating member vand saidzmagnetic eld structure Lfor producing an elastic restoring force therebetween,

said column spring having two critical deectionf values between which its stillness is considerably lower than that prevailing from its free state to Vthe lower ofits'two critical deflection values, said spring stiness changing almost abruptly at the lower of lthe two critical fdeection" values, and means for limiting -tlie relative motion-of said, movable member and magnetic held structure to conne the-deflections of *saidI column spring be-L tween said critical deection values, the relationship between the mass of the magnetic iield structure and the" stiffness of the column spring be- -iween' its critical deflection values beingsuch that the natural frequency ot vibration of the magnetic field structure-is appreciably'below the body, whereby said magnetic field Astructure remains almost stationary during the entire fre-A 9. A device for measuring the vibration velocity'k of a vibrating body, said device comprising a movable magnetic casing adapted -tobe gripped` by the hand of an operator, a permanent magnet within said casing arranged toprovide anv annular air gap between the magnet and casing .as a part `of its magnetic circuit, a movable rod slidably mounted partly within said-casing and having one end protruding therefrom adapted to make con tact' with the vibrating body, an annular coil 'se` cured to the other end of saidrod, said coil being Y disposed in said air gap tocut the magnetic ux tion between the'magnet and col,twolcolumn springs interposed between said 'casing and rod for'producing anelasticrestoring. force therebe-A tween, each column' spring having a practically uniform stiifness from its free state until a critical deiiection value thereof 'at which itsj stiffness al.- most abruptly decreasesvto a considerably lowervalue for deections thereof-till its elasticlimit parts securedinereto and the stiffness of said rbd being such that this structure has a natural frequency of vibration. which is considerably above the lowest-frequency'of ofsaid bc'idyandV nasa narrow' resonance'band in the vicinity of its naturalfrequency of vibration, whereas the relationship between the mass ofthecasing. and magnet andthe stiifness of 'each column spring 4between Vits critical deilectionvalue an'd itselastic limit is'such'thatthenatural frequency .of vibration oi the casing-and magnet is appreciably below the lowest frequency oi' said vibrating body, whereby said 'casing and magnet remain almost stationary duiingtheentire fre,-

quency rangeat which' saidv vibrating body :vi- 1 a movable magneticcasing adapted to be gripped by the hand of anoperator, a permanent magnet.- within said casing arranged to provide an annular air gap between= the lmagnet and casing as a part measuring thevibration ve' of its magnetic circuit, a movable rod partly within said casing and one end thereof'pro. Atruding from the ca'singformaking contactwith the vibrating body, an-annular coil-secured to the 'other'endv'of saidrod, said coil beingdisposed in said air gap, and spring means interposed between s aid rod and said magnet-for producing an elasticrestoring force .therebetween and'for restricting said rod toa, degree of freedom that causes said coil to cut Vthe magnetic flux passing across said'.

air gap withrelative motionfbetween the rod and magnet, therelationship between the mass ofgs'aid rod and the parts secured .fthereto and the stiffness of said rod being such thattliis strueture has. natural frequency of -vibration which is considerably above' the lowest frequency of vi' bi-ation of said vibrating body and has 'a very `narrow resonance band inl the vicinity of its r natural frequency of'vibration, whereas the relationship between the mass of said casing 'and magnet and thestiiness of lsaid spring means being such that the naturalfrequeney of vibra-l tion of said casing and magnet is 'appreciably below the lowest `frequency of vibration of said vibrating body, whereby said casing and magnet remain almost stationary during the entire f-requency range brates.'f' v 11.1119l device of the cli-iss.describea the combination of. two movable structures, at least one column spring interposed between said structures for producing an elastic restoring force therebetween, said column spring having two critical at .which said- 'vibratin'g body 'vi.

deiiection values between'whichits stiffness is.

considerably lower that prevailing from its free state to the lower of its two critical deflection values, said spring stiffness changing almost'ab ruptly at the lower of the two critical deiiection values,"and means for the relative inopassing thereacross in V`response. to relative mo tion 0f Said structures t0 Confine the deectiODS 0f said column spring betweenf saidcritical deiiec-r tion4 values,. the Arelationship between the mass of one of said structures and the stiffness of said column spring between its. critical deflection values being such that this structure remains al-i most stationary while the other structure. is "-l I moving: 12.11A device for measuring thevibrationvelocity in'v a vibrating said device comprising in combination a pair of relatively movable members, one of which is adapted to be placed in arm of the external for'ce acting thereon, and means for producing an electromotive force in response to relative` motion between saidmembers, the member adapted to be placed in contact with the moving -body being relatively light and being relatively rigid in the direction oi Amotion to give it a high natural period of 4vibration .and permit it 'to follow accurately the vibrations of said vibrating body, and the other member hav lng a relatively great mass in comparison with vthe ilrst mentioned member whereby it coopery ates with said column spring to have a relatively vlownatural frequency of vibration causing it to remain substantiallystationary while the other member is in contact with a vibratlngbody.

1 3. In a'deviee for measuring the vibration velocity of a'vibra'ting body, the combination of a pair of lrelatively movable members, a column spring interposed between said movable members,

and means for limiting the motion between said members to potions for any of which said columnspring has its intermediate portion ldeected -to the side so that it is subjectto bending stress y 2,002,181@ spring varies relatively little with7 the moment as well as compressive stress and the variation n longitudinal torce between the-ends of said col- `umn spring' with variation in longitudinal deecti'on thereof is relatively small in comparison withthe `,variation in the longitudinlforce with variation'in longitudinaldeection of a laterally undetiected column spring. v

, 14. A device for' measuring the vibration velocity in a vibrating body, said device vcomprising in Acombination a pairt relatively movable mem-l bers, one ot which is adapted to be'plaeed in con-f' i tact with said moving body, a column spring between said relatively movable members, means for restraining therelative motion of said rela-7 tively movable members to positions Iiforany oi.' which` said column spring is deflected to the side Aand Vthe-*resisting moment of sections of said 4spring-varies relativelylittle lwith the moment.

arm of 'the' externalforc'e acting thereon, and measuring means responsive to the rate oi relative motion between said relatlvely movable members. l

' CHARLES D, GREENTREE.

MILTON S. MEAD.) JR. 

